Volume and heat budgets in the coastal California Current System

The data-assimilating California State Estimate (CASE), a regional implementation of the MITgcm-ECCO 4DVAR system, enables four-dimensional physical budget calculations that reveal new insights about circulation patterns and thermohaline variability in the California Current System (CCS). Coastal volume budgets capture the mean coastal upwelling overturning cell. It is comprised of a vertical branch at the coast and a shallow, cross-shore ageostrophic branch, as well as compensating onshore and alongshore geostrophic transport. The depth and strength of the upwelling overturning cell exhibit a strong seasonal cycle, as does the semiannual intensification of the California Undercurrent. Both have implications for upwelling source water characteristics and are critical to understanding the drivers of the region’s biological productivity. On interannual timescales, CASE shows a persistent warm, downwelling anomaly during the 2014-2015 marine heat wave and anomalous poleward heat advection during the 2015-2016 El Niño event. The latter indicates a tropical influence on the CCS region, driven by a strengthening of the poleward California Undercurrent and a shift in the transported water mass toward warmer and saltier properties. The CASE proves to be an effective tool for monitoring and interpreting the physical variability of the coastal CCS.